It generally is accepted that the capacity for exercise increases as a result of physical training while the process of aging results in a decreased capacity for exercise. Our overall objective of the work described in this proposal is to investigate the effects of aging and physical training on the ultrastructure of the rat heart. Recently, evidence has accumulated to show that the capacity for endurance exercise is related to the mitochondrial characteristics of a tissue. Our specific objective of the work described in this proposal is to characterize mitochondrial distributions in the ventricular apex region of the rat heart. These mitochondrial distributions in combination with measures of fiber diameters, capillary density and capillary to fiber ratios will give us some indication of the aerobic capacity of the cell. To meet the stated objectives, we propose to study the selected morphological characteristics of young, adolescent, mature, old and senescent sedentary and physically trained rats. We plan to use two levels of physical training (short-term intense and long-term low level) and initiate the training regimens at various ages so as to assess the effect of training as a function of the age-at-initiation of training. At preselected ages the animals will be sacrificed and samples of the heart prepared for examination in the electron microscope and/or by light microscopy. Of those tissue samples prepared for electron microscopy, additional "thick sections" will be cut and examined by light microscopy so as to correlate our electron and light microscopy data. BIBLIOGRAPHIC REFERENCES: Cosmas, A.C. and D.W. Edington, Mitochondrial Distributions in Hearts of Male Rats as a Function of Long-Term Physical Training. in Metabolic Adaptation to Prolonged Physical Exercise. (edited by H. Howald and J.R. Poortmans) Birkhauser Verlag, Basel (1975) 390-396.